A plurality of intermediate cross sections and center points thereof are provided for a plurality of target processed portions from an unprocessed portion of a workpiece up to a final target processed portion. By adjusting a relative position between each intermediate cross section of the workpiece W and rollers 11, 12 revolving around the workpiece between neighboring intermediate cross sections, adjusting a revolution diameter of the roller and an angle of its revolution plane, mating the center point, diameter and inclined angle of the revolution plane inside of a revolving locus of the roller, with the center point, diameter and inclined angle of each intermediate cross section of the workpiece, and driving the roller and workpiece relatively to each other, with a part of outer peripheral surface of the roller being always in contact with an outer peripheral surface of the workpiece, a spinning process is performed to change the diameter of the portion to be processed of the workpiece.